Many devices, such as high definition televisions and computer screens, utilize luminescent phosphor as a coating on a display screen for producing images. Such displays typically have an electron source that directs a stream of electrons toward the luminescent phosphor coating. When electrons impinge upon individual phosphor picture elements or pixels on the screen, they cause the energy level of the phosphor to increase to an excited state. When the energy level declines from this excited state, the pixels emit photons. These photons pass through the screen to be seen by the viewer as a point of light. An array of these pixels operating under controlled excitation produce a visible image.
Typically, the cathode ray tube (CRT) has been used as a display for many applications. The CRT has many disadvantages due to its size, power consumption and bulk, that make it unacceptable for many applications, including laptop computers. An emerging technology that overcomes many of the problems of conventional CRT displays is the field emission display (FED). The FED consists of an array of cold cathode emitter tips arranged on a substrate and connected to the negative terminal of a power supply through an addressing scheme. Each of the emitter tips is surrounded by a gate that is connected through an addressing scheme to the positive terminal of the power supply. When an electric field is placed between the gate and the emitter tip, the emitter tips release a stream of electrons. Each of the individual emitter tips or sets thereof is individually addressable so that any combination of them can be activated at a given time. A screen coated with a layer of luminescent phosphor is located opposite the array of emitter tips. When an individual emitter tip is selected, it emits a stream of electrons forming an emission current that impinges upon the corresponding phosphor picture element or pixel on the screen.
Many applications for which the FED is well suited, such as a laptop computer, viewcam finder and head mounted display, require low power consumption. The power consumption of the FED is related to the electric field that must be placed between the gate and the emitter tip in order to produce the required emission current. Very small luminescent phosphor particles can be more efficient than larger particles since they pack more densely on the screen and have larger surface area per unit weight, and thus are more likely to be excited by any single electron.
It is known in the art to produce luminescent phosphor by a multi-step process. In a first step, yttrium oxide is combined with europium oxide and the mixture ground to an average particle size of approximately 5 microns. At this point, the ground particles are not luminescent. The particles can be made luminescent by heating them to a very high temperature (around 1150.degree. C. or higher). However, in addition to becoming luminescent, the phosphor particles grow to an average particle size of about 8 microns. This size is too large for many applications. While these luminescent phosphor particles can be milled to a smaller particle size, this process causes a substantial loss in the luminescence of the particles.
A need exists in the industry for a process to make a luminescent phosphor having both a very small particle size and a high luminescence, as well as a need for products utilizing such luminescent phosphor particles. The present invention fulfills these needs, and provides further related advantages.